The 2nd Generation (2G) and the 3rd Generation (3G) mobile communication networks, such as the Global System for Mobile communications (GSM) and Universal Mobile Telecommunications System (UMTS), provide voice services by way of Circuit Switching (CS), and form good coverage.
The development of radio access network towards packet switching enables the radio access network to bear voice and other services through a Packet Switching (PS) network, and directly access an IP Multimedia Network Subsystem (IMS), and the communication services including voice service are provided by the IMS. With the emergence of various high-speed broadband mobile access modes, the mobile communication standard organization proposed a next-generation network architecture called Long Term Evolution/System Architecture Evolution (LTE/SAE). LTE/SAE is a universal expressing method for evolved packet switched domain networks, wherein LTE represents the radio side, and SAE represents the core network side. This network architecture enables the evolved wireless communication system to provide a higher transmission rate, a shorter transmission delay, and have a lower cost, and meanwhile supports mobility between access systems within the 3rd Generation Partnership Project (3GPP), and mobility between a 3GPP access system and a non-3GPP access system, etc.
With the development of wireless broadband technology and the development of Internet Protocol (IP) technology, the whole network evolves towards the direction of all-IP, and the core network of the communication network evolves towards the IMS.
IMS is an IP-based network architecture proposed by 3GPP, it constructs an open and flexible service environment and is unrelated with access, and it can support multimedia applications and can provide plentiful multimedia services for users.
In the IMS service system, the control layer is separated from the service layer. The control layer does not provide specific services and only provides necessary triggering, routing, charging and other functions for the service layer. The service triggering and controlling functions in the control layer are implemented by the Call Session Control Function (CSCF). CSCF is divided into three types, namely, Proxy-CSCF (P-CSCF), Interrogating-CSCF (I-CSCF), and Serving-CSCF (S-CSCF), wherein the S-CSCF bears the main responsibility and the Interrogating type is optional. The service layer consists of a series of Application Servers (AS), and can provide specific services. The AS may be an independent entity, or may exist in the S-CSCF. The control layer S-CSCF controls service triggering according to the subscription information of the user, invokes the services on the AS, and implements the service functions. The AS and S-CSCF can be called as Server Equipment (SE) collectively. The end-to-end device in the session is called as User Equipment (UE) or terminal, and is responsible for interaction with the user. Some UEs have various modes for accessing the network, including accessing the network through the PS domain of 3GPP, accessing the network through other non-3GPP data domains, and can even access the network through the CS domain. Therefore, both the CS network and PS network (including LTE/SAE) can be used as access means for the IMS network. The user can set up an IMS session through the LTE/SAE or CS, the IMS is used to provide various service controls, and the LTE/SAE or CS is used to provide network access for the user. When the UE sets up various session requests through the IMS, the UE communicates with each entity in the IMS network using the standard Session Initiation Protocol (SIP).
When a UE implements a voice service in the access coverage area of the LTE/SAE, the UE directly accesses the IMS, and sets up an IMS session with a remote user. Due to limitations such as radio spectrum interference, the UE cannot access the LTE/SAE and the CS network of 2G/3G simultaneously, so when the user moves out from the above LTE/SAE coverage area, the network side switches the session of the user to the CS network in order to ensure the continuity of the voice service. Since the process of remote end update in this switching is relatively long, but the process of the UE locally switching the access network is relatively short, it is relatively hard to ensure synchronization if the two processes occur in parallel, and so the voice interruption of the user in this switching is very long. This switching is called as Single Radio Voice Call Continuity (SRVCC).
In the SRVCC scene, i.e., when a user with SRVCC subscription uses a terminal with SRVCC ability, the signaling portion of the session is anchored on an Access Transfer Control Function (ATCF) of the current residing network, and the media is anchored on an Access Gateway (AGW) controlled by the ATCF. Switching is controlled to occur using the ATCF/AGW as an anchor point, which can shorten the time of voice interruption caused by long process of remote end update and thus improve user experience.
FIG. 1 illustrates the architecture of a UE establishing an IMS voice session with a remote end through the LTE/SAE network, and all the network elements are described as follows:
101, a UE is responsible for interaction with the user, and can establish a session in place of the user through different access modes such as PS/CS;
102, a Mobility Management Entity (MME) is the storage place of the user subscription data in the current network, and is responsible for signaling management of the Non-Access Stratum (NAS) from the terminal to the network, tracking under the idle mode of the user and paging management function and bearer management;
103, a Serving Gateway (S-GW) is a gateway from the core network to the wireless system, and is responsible for user-plane bearer from the terminal to the core network, data buffer under the idle mode of the terminal, the function of initiating a service request from the network side, legal wiretap and packet data routing and forwarding function;
a Packet Data Network Gateway (PDN GW or P-GW) is a gateway of an Evolved Packet System (EPS) and the network outside the system, and is responsible for distribution of the IP address of the terminal, charging function, packet filter, and policy application and other functions.
For briefness, S-GW and PDN GW are described collectively.
104, a Home Subscriber Server (HSS) is the permanent storage place of the user subscription data, and is located in the home network to which the user subscribes;
105, an Access Transfer Control Function (ATCF) is responsible for anchoring the signaling portion of the UE session, and controls the Access Gateway (AGW) to anchor the media portion of the user session;
the ATCF is a logic functional entity, and its practical physical position may be in a certain network element, for example, P-CSCF/Interconnection Border Control Function (IBCF), and the like, on a signaling path when the UE performs IMS registration through the visited network LTE/SAE.
An Access Gateway (AGW) is responsible for anchoring the media portion of the UE session under control of the ATCF;
the AGW may be located in the Transition Gateway (TrGW) of the visited place, the PDN GW, the Media Gateway (MGW) of the CS domain or IMS domain, etc., that is, the AGW can be freely deployed depending on practical conditions and not all the cases are described here.
106, a Call Session Control Function (CSCF), it is divided into three types, namely, Proxy, Interrogating and Serving, wherein the Serving-CSCF bears the main responsibility and the Interrogating type is optional. The control layer S-CSCF controls service triggering according to the subscription information of the user, invokes the services on the AS, and implements the service functions.
The I/S-CSCF are described collectively in the figure.
107, a Service Continuity and Consistency Application Server (SCC AS) is an application server for achieving service continuity;
108, a Remote End establishes an IMS session with a UE, and the remote end may be a UE, or a server for providing services, for example a streaming media server; it has no direction relation with the present invention and thus will not be described in detail here;
as shown in FIG. 1, MME/E-UTRAN/SGW/PDN GW are all LTE/SAE network elements, and ATCF/AGW/CSCF/SCC AS are IMS network elements.
When the UE is in the LTE/SAE network coverage area, it firstly performs EPS attachment, and establishes a channel of the UE—an Evolved Universal Terrestrial Radio Access Network (E-UTRAN)—SGW/PDN GW, on which all subsequent IMS signaling and medias are borne and interacted with the IMS network elements.
FIG. 2 is an existing flowchart of session anchoring when a UE performs attachment through LTE/SAE, and then performs IMS registration, and establishes a voice session, which will be described as follows:
Step 201, the UE initiates an EPS attachment request to the MME at the current residing network LTE/SAE;
in this attachment request, the UE informs the MME of whether the UE has SRVCC capability.
Step 202, the MME initiates a location update request to the HSS.
Step 203, the HSS returns a location update response to the MME, where the response carries the LTE/SAE subscription data of related users.
Step 204, the MME returns an attachment acceptance request to the UE.
By now, the attaching process of the UE in the LTE/SAE is over, and the channel of UE—E-UTRAN—SGW/PDN GW is completely established.
Step 205, the UE initiates an IMS registration request in the current residing network, and the registration request message reaches the ATCF.
Step 206, the ATCF distributes a Visited Session Transfer Number-Single Radio (vSTN-SR) for the user.
If the user is a SRVCC user, the vSTN-SR is used for subsequent SRVCC process that possibly happens to the user.
Step 207, the ATCF sends the registration request message to the visited S-CSCF of the user, and the vSTN-SR distributed in step 206 is carried.
The registration request message in this step needs to firstly reach the home I-CSCF of the user, and an S-CSCF for serving the user is determined through interaction between the I-CSCF and HSS, and then the I-CSCF routes the registration request message to the S-CSCF for processing. In order to simplify the illustration and description, here the I-CSCF and S-CSCF are collectively descried, and the related flow of the HSS is omitted.
Step 208, the S-CSCF initiates a server registration request to the HSS, where the vSTN-SR is carried.
Step 209, the HSS returns a server registration response to the S-CSCF, where related user data are carried.
Steps 210-211, after completely processing the registration request, the S-CSCF returns a registration response message to the UE through the original path.
Step 212, after completely processing the registration response message of the user, the S-CSCF performing the third-party registration to the corresponding application server (AS) in place of the UE if needed according to the initial Filter Criteria (iFC).
If the user is a SRVCC user, the S-CSCF needs to perform the third-party registration to the SCC AS in place of the UE, where the vSTN-SR in step 207 is carried.
Steps 212a-212b, the vSTN-SR is loaded to the LTE/SAE network element (MME) through an HSS promotion process.
During the subsequent process of triggering SRVCC, the vSTN-SR is transmitted to the CS network element by the LTE/SAE network element; or the vSTN-SR is transmitted to the corresponding LTE/SAE and CS network element through other paths so as to achieve SRVCC.
Step 213, the UE initiates an IMS session, and the session request reaches the ATCF, where the Session Description Protocol (SDP) information of the UE, such as, IP address, media stream coding format and port number, etc. is carried.
Step 214, the ATCF decides whether to anchor the media portion of the voice call initiated by the user, or interacts with the network element of the home IMS domain of the user, such as S-CSCF and SCC AS, etc., to decide whether to anchor the media portion of the voice call initiated by the user.
If the ATCF decides to anchor the media portion of the session, the ATCF will distribute an AGW for the session and anchor the media on the AGW.
Step 215, according to the decision result in step 214, the ATCF anchors or anchors the session, and establishes a voice connection with the home domain IMS of the user, and the remote UE.
Step 216, the ATCF returns a session establishment response to the UE.
According to whether to anchor the call or not, the SDP information of the remote UE or the AGW is carried in step 216.
In a practical application process, the session anchored on the ATCF, and the media of the same session anchored on the AGW are not used in many cases, which cause resource waste of the network side.